In this program I will continue the study of sulfur amino acid metabolism in mammals. Three broad approaches will be utilized. We have developed assays for six of the enzymes involved in the conversion of methionine to homocysteine (S-adenosylmethionine synthetase, S-adenosylhomocysteine hydrolase), the conversion of homocysteine to cysteine (cystathionine synthase, cystathionase) and the remethylation of homocysteine (betaine homocysteine methyltransferase and N5 methyltetrahydrofolate homocysteine methyltransferase). Employing these assays, we will evaluate the effect of dietary and hormonal factors on the tissue (liver and other organs) content of the various enzymes. Concurrently, the enzymes will be purified and their kinetic properties determined with particular reference to regulation by intermediates of the pathway as well as by other normal and abnormal metabolites. Finally, I will attempt to correlate these in vitro evaluations of the regulation of enzyme content and activity with a study of the effect of the same dietary and hormonal factors on the metabolism of methionine in the intact rat liver. In the latter experiments, we will use techniques of isolated organ perfusion as well as studies in the whole animal. Due to the essential role of methionine in human nutrition, these studies have a broad relevance. They are particularly significant in the areas of (1) Homocystinuria and other disorders of sulfur amino acid metabolism; (2) Alcoholic liver disease which in the experimental animal is reversed, in part, by methionine; (3) chronic liver disease of many etiologies which are associated with hypermethioninemia and possibly (4) Neuropsychiatric disorders.